Manufacturing a solar cell with backside contacts

ABSTRACT

A solar cell involving a silicon wafer having a basic doping, a light-receiving front side and a backside, which is provided with an interdigital semiconductor pattern, which interdigital semiconductor pattern has a first pattern of at least one first diffusion zone having a first doping and a second pattern of at least one second diffusion zone, separated from the first diffusion zone(s) and having a second doping that differs from the first doping, wherein each second diffusion zone is arranged along the sides of at least one groove extending from the backside into the silicon wafer.

The invention relates to a method of manufacturing a solar cell withbackside contacts.

A solar cell comprises a silicon wafer having a light-receiving frontside and a backside. The silicon wafer is provided with a basic doping,wherein the basic doping can be of the n-type or of the p-type. Thesolar cell is usually provided with metallic contacts on thelight-receiving front side as well as on the backside to carry away theelectric current produced by the solar cell. Especially the metalcontacts on the light-receiving front side pose a problem in regard tothe degree of efficiency, since the metal covering causes shading of theeffective area of the solar cell. Although one optimises the metalcovering so as to reduce the shading, a metal covering of approximately10% remains unavoidable since the metallization has to occur in a mannerthat keeps the electrical losses small. For the metal contacts on thebackside the danger of shading does not occur, however, for contactingan optimisation between the electrical losses and the costs for applyingthe metal contacts at the backside must be achieved.

There are solar cells where both contacts are provided on the backsideof the solar cell, so that the solar cell is not shaded through themetal contacts. An example of such a cell is given in Internationalpatent application publication No. 02/23 639. However, manufacturingsuch solar cells with contacts only on the backside is very elaborateand it involves numerous masking, etching and cleaning process steps.Moreover, the metallization structures must be exactly aligned. Therelatively large costs of manufacturing the solar cells with backsidecontacts have prevented large-scale implementation of these moreefficient solar cells.

It is an object of the present invention to provide a solar cell withbackside contacts that is mechanically strong and wherein recombinationof charge carriers is reduced. It is a further object of the inventionto provide a solar cell with backside contacts that, in comparison withthe known cells, can be manufactured just as reliably but in a morecost-effective way.

To this end the solar cell according to the present invention comprisesa silicon wafer having a light-receiving front side and a backside,wherein the silicon wafer has a basic doping, and is, at its backside,provided with an interdigital semiconductor pattern, which interdigitalsemiconductor pattern comprises a first pattern of at least one firstdiffusion zone having a first doping and a second pattern of at leastone second diffusion zone, separated from the first diffusion zone(s)and having a second doping that differs from the first doping, whereineach second diffusion zone is arranged along the sides of at least onegroove extending from the backside into the silicon wafer.

To carry away the electric current produced by the solar cell, theinterdigital semiconductor structure is provided with an interdigitalcontacting structure.

The invention further relates to a method of manufacturing a solar cell,which method comprises providing a silicon wafer having alight-receiving front side and a backside with a basic doping, andproviding the silicon wafer at its backside with an interdigitalsemiconductor pattern, wherein providing the interdigital semiconductorpattern comprises the steps of:

-   -   (a) applying to the backside a doping paste containing a first        dopant to obtain a pattern of at least one area covered with        doping paste;    -   (b) drying the doping paste;    -   (c) producing a pattern of at least one first diffusion zone        having a first doping by forcing at elevated temperature in a        free oxygen-containing atmosphere the first dopant to diffuse        into the silicon wafer, and simultaneously producing a silicon        oxide layer;    -   (d) etching part of the silicon oxide layer by applying a first        etching agent in areas between the first diffusion zones, and        removing the first etching agent and the etched silicon oxide        layer to obtain a pattern of silicon oxide rims;    -   (e) etching part of the silicon wafer by applying a second        etching agent between the silicon oxide rims, and removing the        second etching agent and the etched silicon to obtain a pattern        of at least one groove;    -   (f) producing a second diffusion zone having a second doping on        the sides of each of the at least one groove, wherein the second        doping differs from the first doping; and    -   (g) removing the doping paste and the remainder of the silicon        oxide layer to obtain the interdigital semiconductor pattern.

To manufacture a solar cell wherein the electric current produced by thesolar cell can be carried away, the method further comprises providingthe interdigital semiconductor structure with an interdigital contactingstructure, which comprises applying a passivating layer on the surfacesof the silicon wafer and the diffusion zones; applying metallizationlayers on the passivating layer, wherein each metallization layerextends along a diffusion zone; and producing electric contacts byfiring the metallization layers.

The invention allows for the structured creation of diffusion zonessince the pastes can be applied to the silicon wafer of the solar cellsthrough screen-printing. Thus, any structure necessary for themanufacturing of solar cells can be formed through a simple, reliablyreproducible and cost-effective manner since through the structuredapplication of pastes in a comparatively simple manner an exactalignment of the individual zones of the solar cell can be achieved.

Through the application of screen printable doping and/or etching pastesin manufacturing of diffusion zones in the silicon wafer of a solarcell, a solar cell with backside contacts can be manufactured in asimple and cost-effective manner. The designs of the screens that wereused for producing the first diffusion zone(s) and for etching thesilicon oxide layer can be used for the screens used to apply themetallization layers. Thus, the invention removes the economicdisadvantages were related to the currently required process steps forthe manufacturing of this type of solar cells.

Through a suitable coordination of the process steps an especiallycost-effective way for the manufacturing of a solar cell with backsidecontacts can be found.

The invention will now be described in more detail by way of example onthe basis of this exemplary embodiment with reference to the Figures inwhich:

FIG. 1 shows schematically and not to scale part of a cross-section of asolar cell according to the present invention; and

FIGS. 2 through 8 show schematically steps of the process ofmanufacturing the solar cell according to the present invention.

Reference is now made to FIG. 1 showing schematically and not to scalepart of a cross-section of a solar cell 1 according to the presentinvention.

The solar cell 1 comprises a silicon wafer 3 having a light-receivingfront side 4 and a backside 6. The silicon wafer 3 has a basic doping,which is in this case a p-type doping.

At the backside 6, the silicon wafer 3 is provided with an interdigitalsemiconductor pattern that comprises a first pattern of at least onefirst diffusion zone 9 having a first doping, and a second pattern of atleast one second diffusion zone 10. The second diffusion zones 10 areseparated from the first diffusion zones 9, and they have a seconddoping that differs from the first doping. Each second diffusion zone 10is arranged along the sides of at least one groove 12 extending from thebackside 6 into the silicon wafer 3. Suitably the number of grooves isin the range of from 1 to 100 grooves per centimetre width of the wafer,and the width of a groove is suitable in the range of from 0.05 to 5millimetre, and the width of the rim between adjacent grooves is also inthe range of from 0.05 to 5 millimetre. Suitably the grooves areparallel to each other.

Suitably, the interdigital semiconductor structure is provided with aninterdigital contacting structure, wherein the first diffusion zones 9are provided with a first contacting structure 13, and the seconddiffusion zones 10 are provided with a second contacting structure 14 toallow carrying away the electric current produced by the solar cellduring normal operation. The interdigital contacting structure forms thebackside contacts.

Suitably, the first doping of the first diffusion zones 9 is of the sametype as the basic doping of the silicon wafer 3. Consequently, thedoping of the second diffusion zones 10 differs from the basic doping.

An advantage of the solar cell according to the present invention isthat the thickness of the silicon wafer 3 at the location of the grooves12 (the thin wafer section) is smaller than the original thickness ofthe silicon wafer 3 at the location of the first diffusion zones 9. Thusat the location of the second diffusion zones 10, which suitably have adoping that is different from the basic doping the thickness is small,which reduces the possibilities for recombination of carriers. And thethickness at the location of the first diffusion zones is large toprovide mechanical strength to the solar cell 1 of the presentinvention.

The grooves 12 extend into the silicon wafer 3 so as to form a thinwafer section, wherein the thickness of the thin-wafer section issuitably between 30 to 60% of the thickness of the silicon wafer 3 or inthe range of between 50 and 150 micrometre, whichever is the smallest.

The front side 4 is suitably provided with an anti-reflection coating 15and the backside, between the contacting structures 13 and 14 isprovided with an anti-reflection coating 17. The anti-reflectioncoatings 15 and 17 also serve to passivate the surface of the siliconwafer 3. Suitable materials for the anti-reflection coating are siliconoxide and silicon nitride or a mixture of silicon oxide and siliconnitride.

To provide sufficient electrical insulation between the diffusion zonesof different types, the size of the separation 18 between a first and asecond diffusion zone 9 and 10 is suitably greater than the thickness ofthe second diffusion zone 10, and suitably greater than the sum of thethicknesses of the first and second diffusion zones 9 and 10.

The method of manufacturing a solar cell according to the presentinvention will now be discussed with reference to FIGS. 2 through 8.Features already discussed with reference to FIG. 1 will get the samereference numerals.

As with other manufacturing processes, the starting point for themanufacturing of a solar cell with backside contacts according to theinvention is a sawn silicon wafer 3 with a suitable p-type or n-typebasic doping. The thickness of the silicon wafer 3 can be freely chosendepending on the solar cell design. The surface layer can be damaged bythe sawing step, and this damage is removed by etching. Depending on thesolar cell design, additional preparatory process steps may follow, forexample a process step in which the silicon wafer 3 undergoes textureetching as described in German patent application publication No. 198 11878.

The silicon wafer 3 has a light-receiving front side 4 and a backside 6.The first step of providing the backside 6 of the silicon wafer 3 withan interdigital semiconductor pattern comprises applying to the backside6 a doping paste 20 containing a first dopant to obtain a pattern of atleast one area covered with doping paste 20 (see FIG. 2). The dopingpaste 20 is suitably applied by means of screen-printing.

Subsequently the doping paste 20 is dried.

Reference is now made to FIG. 3. The next step is producing a pattern ofat least one first diffusion zone 9 having a first doping by forcing atelevated temperature in a free oxygen-containing atmosphere the firstdopant to diffuse from the doing paste 20 into the silicon wafer 3, andsimultaneously producing silicon oxide layers 21 and 22 at thelight-receiving front side 4 and the backside 6.

The first dopant can be boron, aluminium, gallium, or indium to obtainp-doped first diffusion zones 9, or phosphorus, arsenic or antimony toobtain n-doped first diffusion zones 9. The elevated temperature issuitably between 800° C. and 1 200° C. (for example between 900° C. and1 200° C. when the dopant is boron, and between 800° C. and 1 000° C.when the dopant is phosphorus).

The next step is shown in FIG. 4. This step comprises etching part ofthe silicon oxide layer 22 by applying an etching agent 25 in areasbetween the first diffusion zones 9, and removing the etching agent 25and the etched silicon to obtain a pattern of silicon oxide rims 26. Theetching agent is suitably an etching paste that is applied by means ofscreen-printing, wherein the active ingredient is an aqueous acidicsolution.

The silicon oxide rims 26 are now used as a mask for producing thegrooves 12 (see FIG. 1) as will be explained with reference to FIG. 5.Between the silicon oxide rims 26, a second etching agent 27 is applied.When the etching has reached the required depth, the second etchingagent and the etched-away parts of the silicon wafer 3 are removed toobtain the pattern of at least one groove 12. The etching agent issuitably an aqueous alkaline solution that is so selected that siliconoxide is not etched away.

Reference is now made to FIG. 6, showing the result of the next step.This step involves producing second diffusion zones 10 having a seconddoping on the sides of each of the at least one groove 12, wherein thesecond doping differs from the first doping. Because the etching agentin the previous step was applied between the first diffusion zones 9,after removing the etching agent, rims 26 of silicon oxide remain, andthese rims are responsible for the separation 18 (see FIG. 1) betweenthe first and second diffusion zones 9 and 10. The rims 26 areprotective zones, which forms a mask for the diffusion of the seconddoping.

Suitably, the diffusion of the second dopant is done from a gaseousphase, the dopant can be phosphorous or boron.

The last step of the method according to the present invention isremoving the doping paste 20 and the rims 26 of the silicon oxide layerto obtain the interdigital semiconductor pattern 9, 10 at the backside 6of the silicon wafer 3. A suitable etching agent to remove the dopingpaste 20 and the rims 26 is diluted hydrofluoric acid. The result isshown in FIG. 7.

The interdigital semiconductor pattern comprises the first pattern firstdiffusion zones 9 having a first doping, and the second pattern seconddiffusion zones 10 arranged along the sides of the grooves 12. Thesecond diffusion zones 10 are separated from the first diffusion zones9, and they have a second doping that differs from the first doping. Anadvantage of the process according to the invention that the etchingstep is carried out such that edge isolation between the diffusion zones9 and 10 is created automatically so that an additional edge isolationstep necessary for this, for example, through plasma etching, can beomitted.

In order to carry away the electric current produced during normaloperation of the solar cell, metal contacts are applied on theinterdigital semiconductor pattern. Prior to this step, the surfaces ofthe silicon wafer 3 can be passivated by applying a suitableanti-reflection coating that serves as well as a passivation layer, suchas silicon nitride, silicon oxide or a mixture of silicon nitride andsilicon oxide.

The surfaces 4 and 6 are provided with a silicon oxide anti-reflectioncoating 15 and 17 (see FIG. 8).

Then metallization layers 30 and 31 are applied on the anti-reflectioncoating 17 that is applied on the back surface 6, wherein eachmetallization layer 30 and 31 extends along a diffusion zone 9 and 10.Subsequently the electric contacts 13 and 14 (see FIG. 1) are obtainedby firing the metallization layers 30 and 31. The paste used for themetallization layers can be doped or free from dopant. Suitably thepaste is free from dopant, because the diffusion zones already providethe ohmic contact. Using a paste that is free from dopant has theadditional advantage that the same paste is used for the twometallization layers 30 and 31.

The metallization layers 30 are suitable applied by means ofscreen-printing using screens of the same design as the screens thatwere used to apply the doping paste 20. And the metallization layers 31are suitably applied by means of screen-printing using screens of thesame design as the screens that were applied to apply the etching agent25 (see FIG. 4). In this way the areas later forming the electriccontacts 13 and 14 can be easily aligned with the diffusion zones 9 and10.

Alternatively the designs of the screens used to print the metallizationlayers 30 and 31 can be combined into one so that both metallizationlayers can be screen-printed in one step. In this case the differentcontacts are already aligned with respect to each other and thereforethere are no problems with cross-contacting or cross-contamination. Themetallization paste used in the alternative process is free from dopant,so that the same paste is used for both metallization layers 30 and 31.

The basic doping of the silicon wafer can be a p-type or an n-type, thedoping of the first diffusion zones 9 can be a p-type or an n-type, andthe doping of the second diffusion zones 10 is then either an n-type ora p-type.

Suitably, the doping of the first diffusion zone is the same as thebasic doping, so as to form first diffusion zones 9 having a largerconcentration of the carriers pertaining to the doping than the siliconwafer 3. Such a concentration difference of carriers of the same kind isreferred to as a back surface field, which is in the solar cellaccording to the present invention a local back surface field, becauseit is not continuous along the backside 6. In this case, the doping ofthe second diffusion zones 10 differs from the basic doping and a p-n oran n-p junction is formed at the interfaces.

Depending on the design of the solar cell and its application, differentconcentrations and penetration depths of the dopant in the diffusionzones 9 and 10 can be specified.

Because of the already existing high doping in the diffusion zones 9 and10, an un-doped paste, for example an un-doped silver paste, can be usedfor the two metal contacts 13 and 14. However, in case of different orinsufficient doping, doped metallization pastes can be used that havebeen adjusted for the contacting of the respective areas.

The individual solar cells manufactured according to the presentinvention can be integrated to form a solar module. To this end thebackside contacts of neighbouring cells are joined by suitable bondingmaterial to form a series connection or a parallel connection.

1. A solar cell comprising a silicon wafer having a light-receivingfront side and a backside, wherein the silicon wafer has a basic doping,and is, at its backside, provided with an interdigital semiconductorpattern, which interdigital semiconductor pattern comprises a firstpattern of at least one first diffusion zone having a first doping and asecond pattern of at least one second diffusion zone, separated from thefirst diffusion zone(s) and having a second doping that differs from thefirst doping, wherein each second diffusion zone is arranged along thesides of at least one groove extending from the backside into thesilicon wafer.
 2. The solar cell according to claim 1, wherein theinterdigital semiconductor structure is provided with an interdigitalcontacting structure.
 3. The solar cell according to claim 1, whereinthe first doping is of the same type as the basic doping.
 4. The solarcell according to claims 1, wherein the groove(s) extend into the waferso as to form a thin wafer section.
 5. The solar cell according toclaims 1-4, wherein the size of the separation between a first and asecond diffusion zone is greater than the thickness of the seconddiffusion zone.
 6. A method of manufacturing a solar cell, which methodcomprises providing a silicon wafer having a light-receiving front sideand a backside with a basic doping, and providing the silicon wafer atits backside with an interdigital semiconductor pattern, whereinproviding the interdigital semiconductor pattern comprises the steps of:(a) applying to the backside a doping paste containing a first dopant toobtain a pattern of at least one area covered with doping paste; (b)drying the doping paste; (c) producing a pattern of at least one firstdiffusion zone having a first doping by forcing at elevated temperaturein a free oxygen-containing atmosphere the first dopant to diffuse intothe silicon wafer, and simultaneously producing a silicon oxide layer;(d) etching part of the silicon oxide layer by applying a first etchingagent in areas between the first diffusion zones, and removing the firstetching agent and the etched silicon oxide layer to obtain a pattern ofsilicon oxide rims; (e) etching part of the silicon wafer by applying asecond etching agent between the silicon oxide rims, and removing thesecond etching agent and the etched silicon to obtain a pattern of atleast one groove; (f) producing a second diffusion zone having a seconddoping on the sides of each of the at least one groove, wherein thesecond doping differs from the first doping; and, (g) removing thedoping paste and the remainder of the silicon oxide layer to obtain theinterdigital semiconductor pattern.
 7. The method according to claim 6,which method further comprises providing the interdigital semiconductorstructure with an interdigital contacting structure, which comprisesapplying a passivating layer on the surfaces of the silicon wafer andthe diffusion zones; applying metallization layers on the passivatinglayer, wherein each metallization layer extends along a diffusion zone;and producing electric contacts by firing the-metallization layers. 8.The method according to claim 6, wherein the doping paste in step (a) isapplied by means of screen-printing.
 9. The method according to claims6, wherein the etching agent is an etching paste, which is applied bymeans of screen printing.
 10. The method according to claims 6-9,wherein forcing the dopant to diffuse into the silicon wafer in step (c)is carried out at a temperature in the range of from 800° C. to 1 200°C.
 11. The solar cell according to claim 5, wherein the size of theseparation between a first and a second diffusion zone is greater thanthe sum of the thicknesses of the first and second diffusion zones.